The present invention relates generally to a method of geophysical exploration and more particularly to a method for estimating formation lithology employing measures of formation anisotropy.
In the continuing search for oil and gas, it is oftentimes useful to be able to estimate formation lithology since formation lithology can be a guide to the location of oil- and gas-bearing formations. Generally, formation lithology can be characterized as carbonate, sandstone, shale or combinations thereof. Various methods for obtaining estimates of subsurface formation lithology have been proposed. By way of example, Sondergeld, et al., in U.S. Pat. No. 4,692,910, describe a method whereby seismic wave velocities of propagation in subsurface formations can be used to obtain estimates of formation lithologies. Additionally, U.S. Pat. No. 4,554,649 Herkenhoff et al., describe methods for obtaining estimates of subsurface formation lithologies whereby range dependent amplitude variations of events recorded in seismic signals can be employed to provide estimates of subsurface formation lithologies.
More recently, exploration geophysicists have noted that certain subsurface formations can exhibit velocity anisotropy. For example, see the discussions by K. Helbig "Systematic Classification of Layer-Induced Transverse Isotropy," Geophysical Prospecting Volume 29 (1981), page 550-577, and L. Y. Brodov, et al., "Some Results of the Experimental Study of Seismic Anisotropy of Sedimentary Rocks Using Different Types of Waves," Geophysics J. R. Astronomy Society, Volume 76 (1984), page 191-200, as well as the following commonly assigned applications: Alford in U.S. Pat. Application No. 633,160, or Alford, et al., in U.S. Application Pat. No. 633,163; additionally, Sondergeld, et al. in U.S. Pat. Application No. 913,210 and Sondergeld, et al. in U.S. Pat. Application No. 108,847.
In spite of the advances in seismic exploration as well as the desirability of estimating formation lithology, geophysicists have failed to develop techniques for estimating formation lithology from measures of formation anisotropy.